Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere.
Evaporative leaks may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system may be isolated at an engine-off event. The pressure in such a fuel system will increase if the tank is heated further (e.g. from hot exhaust or a hot parking surface) as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and leaks identified based on expected vacuum development or expected rates of vacuum development.
Current federal regulations require that the EONV test has a completion frequency of 52%, and further require that the EONV test is executed following roughly half of all drive cycles. However, not all drive cycles are conducive to successful EONV tests. For example, a short drive cycle may reject less heat to the fuel tank than a longer drive cycle, and a subsequent EONV test will not be as robust. A vehicle that typically makes trips that comprises multiple shorter trip legs may have a low completion frequency, as the engine may be restarted prior to the completion of an EONV test. Further, hybrid vehicles that operate alternately in a combustion mode and an engine-off mode may lose heat from the fuel tank during time periods when the engine is off, decreasing the accuracy of a subsequent EONV test.
The inventors herein have recognized the above problems and have developed systems and methods to at least partially address them. In one example, a method for a vehicle, comprising: during a first condition, closing a canister vent valve responsive to an engine-off event without initiating an engine-off natural vacuum test; during a second condition, following the first condition, closing the canister vent valve responsive to a vehicle-off event; and then initiating an engine-off natural vacuum test. By closing the canister vent valve during an engine-off event, heat rejected to the fuel tank may be retained during the engine-off event. By initiating the engine-off natural vacuum test at a subsequent vehicle-off event, the likelihood of an engine restart disrupting the engine-off natural vacuum test may be decreased. In this way, the engine-off natural vacuum test will have an increased completion percentage.
In another example, a method for a vehicle, comprising: at a first vehicle-off event, determining a likelihood of a next vehicle-on event during a predetermined window; and closing a canister vent valve if the likelihood is above a threshold. The likelihood of the vehicle-on event may be determined based on driving habits of the vehicle operator. In this way, the vehicle may be trained through machine learning to anticipate when the vehicle is likely to be stopped for a short period of time followed by a vehicle-on event. Engine-off natural vacuum tests may be performed when the vehicle-off duration is likely to be greater than the duration of the test.
In yet another example, a method for a hybrid vehicle, comprising: during a first condition, closing a canister vent valve while operating the hybrid vehicle in an engine-off mode; and opening the canister vent valve responsive to the hybrid vehicle entering a combustion mode. By closing the canister vent valve during engine-off operation, heat dissipation from the fuel tank may be reduced during engine-off vehicle conditions. In this way, the entry conditions for an engine-off natural vacuum test may be met more frequently. Further, by retaining heat in the fuel tank, the robustness of the engine-off natural vacuum test may be increased for vehicles with limited engine run time.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.